Does not set accept-encoding header when zuul proxies request if already present.  Fixes #2998 (#3011)

This fixes problems in testing while maintaining backwards compatibility.

Allows upgrade of zuul to 1.3.1

Fixes #2979

* Fixed: remove misplaced StringEncoder setting place when dealing with protobuf

* test SpringEncoder when protobuf is not in classpath

* Fixed content length handling issues. (#2894)

* Added test setup and cleanup for RequestContext. (#2894)

* Retryable Clients should validate the service server and throw an exception if necessary

* fixing spelling error

* RetryableRibbonLoadBalancingClient should throw a ClientException which can be handled by the RibbonRoutingFilter

* moving service instance validation and service instance chooser to the AbstractLoadBalancingClient

* Updating RetryCallback within RetryableRibbonLoadBalancingHttpClient to handel Exception

Allowing RibbonLoadbalancedRetryPolicy to update ServerStats for circuit tripping exceptions (#2483)

* Allowing RibbonLoadbalancedRetryPolicy to update ServerStats for circuit tripping exceptions

Prevent RibbonRoutingFilter throws exceptions when Http status codes are non-standard(Against the 1.4.x branches) (#2671)

* This PR fixes #2639 and against the 1.4 branches

Fixes #2625

Set hystrix timeout for Zuul routes based on whether the hystrix command timeout, default timeout, or ribbon timeouts are set.  Fixes #2606 (#2633)

* InputStream should always be closed after reading contents from upstreams

* Fixes double encoding when creating secure URLs.  Fixes #2478.

* Add support loadBalancerKey in RibbonRoutingFilter

* Add documentation about how to provide a loadBalancerKey to Ribbon

* Add integrationtest for RibbonRoutingFilter's loadBalancerKey support

* Adds backoff policies when retrying requests.  Fixes #2195

* fixing final variable

* Add new Zuul filters endpoint and refactor routes endpoint
* Refactor FiltersEndpoint
- Extend from AbstractEndpoint and simplify code
- Move tests classes into package endpoint
- Remove usage of Lombok
* Conditionally enable Zuul routes and filters endpoints
* Provide FilterRegistry to FiltersEndpoint via constructor

See https://github.com/spring-cloud/spring-cloud-commons/issues/265

Modfied RibbonCommandContext to make request entity resettable. fixes gh-892

According to:

https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/bind/annotation/RequestMapping.html#headers--

the `headers` field can take several formats:

1. Require the header with a concrete value: `"My-Header=myValue"`
2. Require the header with any value: `"My-Header"`
3. Require the header is absent: `"!My-Header"`
4. Require the header with a negated value : `"My-Header!=myValue"`

This fixes `parseHeaders` for cases 2-4 (which previously threw a `StringIndexOutOfBoundsException`).

Fixes gh-1778.

Cause can now be propagated to fallback provider in order to respond accordingly to a exception type.
New interface is introduced in order not to break any existing falback providers.